The present invention relates to a control apparatus for driving a motor and, more particularly, to a control apparatus for a motor which requires an accurate torque output.
This patent application is based on Japanese Patent Application No. 10-280112, filed Oct. 1, 1998, the entire content of which is incorporated herein by reference.
FIG. 1 shows the general arrangement of an IGBT inverter apparatus.
FIG. 1 shows a power driving apparatus based on vector control with velocity feedback and current feedback control.
As shown in FIG. 1, an input velocity reference signal .omega.r* and a velocity feedback signal .omega.' calculated by a velocity detector 1, primary magnetic flux angle calculator 2, and differentiator 3 are feedback-calculated. The calculation result is converted into a torque reference signal Tr by a velocity controller 4. A new torque reference signal is calculated by adding the torque reference signal Tr to a torque reference signal Tr* input from a main host apparatus. The calculated torque reference signal is divided by a secondary magnetic flux reference .PHI..sub.2 * to obtain a q-axis torque reference signal Iq*.
On the other hand, a field weakening controller 5 and magnetic flux saturation pattern generator 6 are used to calculate a d-axis current reference signal Id* from the velocity feedback signal .omega.'. The calculated q-axis torque reference signal Iq* and d-axis current reference signal Id* and d- and q-axis current feedback signals Id' and Iq' are feedback-calculated to generate final current reference signals Id* and Iq*, respectively.
The current reference signals Id* and Iq* are output from current controllers 7 and 8 as voltage references Vd* and Vq*, respectively.
A coordinate conversion/PWM conversion device 9 outputs element gate ignition pulse instructions Gu, Gv, and Gw on the basis of the voltage references Vd* and Vq* output from the current controllers 7 and 8.
On the basis of the element gate trigger pulse instructions Gu, Gv, and Gw output from the coordinate conversion/PWM conversion device 9, a power converter 10 converts a DC voltage Vdc supplied from a DC current source into a desired AC voltage Vac and outputs the voltage. The power converter 10 supplies a desired current to a motor 11 to drive it.
In the above arrangement, for a d-axis field current Id as a motor field component in vector control, the secondary magnetic flux reference .PHI..sub.2 * is calculated in accordance with the velocity feedback signal .omega.' using a certain field pattern in the field weakening controller 5.
The field pattern in the field weakening controller 5 is determined by the motor connected to the motor control apparatus and set as a fixed value. When a velocity feedback signal .omega. having a certain magnitude is input, the field current component Id in the d-axis of the motor is calculated in accordance with the field pattern.
On the basis of a primary magnetic flux angle .theta.r calculated by the primary magnetic flux angle calculator 2 and a slip angle .theta.s calculated by vector control, a secondary magnetic flux angle .theta.o necessary for vector control such as 2-to 3-axis conversion or 3- to 2-axis conversion is obtained. For this reason, calculation of the secondary magnetic flux angle .theta.o produces a delay.
In the above-described conventional motor control apparatus, in a region where the motor load as the load of the motor control apparatus is low, a motor current vector I.sub.1 in vector control stays close to the d-axis as an excitation current component, as shown in FIG. 2, and we have EQU I.sub.1 =I.sub.d (1)
When the motor current I.sub.1 stays near the d-axis as the field current component, the ratio of an error component caused by the conversion/calculation delay in the current feedback coordinate converter 13 and coordinate conversion/PWM conversion device 9 in the current feedback loop to the q-axis torque current component signal Iq as the torque component in vector control is high in the low-load mode, resulting in degradation in torque accuracy.